Page 141 - Bridge and Highway Structure Rehabilitation and Repair

116 SECTION 1 ADMINISTRATIVE ISSUES

It is important that effective communication be maintained and weekly meetings be held
between the consultant, client, and the contractor to resolve immediately the imminent day-to-
day issues. Full-time two-way communication needs to be maintained by full use of the modern
media such as fax, e-mail, letter, and cell phone.
3.18.6 Hazard Control Principles

Engineering sense requires that the following intuitive approach be adopted by the manage-
ment team:

1. Recognize defects and deficiencies through inspection and monitoring.
2. Define preventive action and estimate of costs.

3. Allocate of resources to match the needs.
4. Assign responsibility for preventing action such as redesign and repairs.
5. Adopt quality assurance and quality control procedures.
6. Implement in a timely manner.

3.19 THE STUDY OF MODES OF FAILURE

3.19.1 Mechanics of Failure

In practice, failures occur in different forms in a material. They are likely to be different
for steel, concrete, and timber bridges. Physical forms of failure can be seen as infi nitely large
deformation and metallurgical disintegration of elements. It can be localized cracking without
causing collapse or discontinuity and total separation of a bridge component. Common types
of failures are:
1. Yielding (metals—crushing, tearing or formation of ductile or brittle plastic hinges).
2. Buckling (metals, web buckling).
3. Crushing (concrete).
4. Fracture and fatigue (metals and concrete—reduced material resistance, local hairline cracks,
minor or major cracks in the deck slab, girders or abutments, reversal of stress in welds and
connections, vibrations).
5. Rupture (shearing).
6. Large deformations (metals and concrete—impact, sway, violent shaking during seismic
events, erosion of soil in floods, settlement due to expansive soils).

7. Stress concentrations (concrete deck slabs with sharp skew).
8. Corrosion (metals and concrete—reduction in material area).

3.19.2 Forms of Failure

For composite beams, plastic hinges form at midspan and for continuous beams at supports.

Visible tension yielding occurs in the bottom flange and at the continuous supports at the top
flange accompanied by cracking at the surface of the slab.

In prestressed concrete beams, collapse may occur due to principal tensile stress at anchor-
ages or breakage of the corroded strands.
3.19.3 Physical Causes

1. Usually it is a combination of more than one type of force that causes failure. For example,
dead load stress must always be combined with one or more external transient forces to
apply a compound critical stress. If dead load stress is already high and approaching the
elastic limit, any applied force or stress will exceed the allowable limit and lead to failure.